MySQL index (1)

MySQL Indexes

The concept is introduced

What is index

An index is a data structure that stores records in a database in a special form . Through the index , Can significantly improve the efficiency of data query , So as to improve the performance of the server .

An index is an ordered list , In this list are stored the value of the index and the physical address of the row containing the data of this value . When the database is very large , Indexing can greatly speed up queries , This is because after using the index, you can locate the data of a row without scanning the whole table , Instead, we first find the corresponding physical address of the row data through the index table, and then access the corresponding data .

Indexes , It's not MySQL Database specific mechanism , There will be similar and different implementations in relational databases . Here we are just discussing MySQL Index implementation in database .

in fact , be supposed to MySQL The index is actually not accurate . Because in MySQL in , Indexes are implemented at the storage engine level, not at the server level . This means that the index we are talking about is exactly InnoDB Engine or MyISAM Engine or other storage engine .

So the index is even in MySQL There is no uniform standard in , The indexes implemented by different storage engines work differently . Not all storage engines support the same type of index , Even multiple engines support the same type of index , The underlying implementation may be different .

Why index is needed

The index seems to be adding a to the database 「 Catalog page 」, Can easily query data . But that's all the index does , Why do you need to spend a lot of time building and optimizing indexes ？

After using the index, you can locate the data of a row without scanning the whole table , Instead, we first find the corresponding physical address of the row data through the index table, and then access the corresponding data . This method naturally reduces the amount of data that the server needs to scan the database when responding .

More Than This , The scope of the database when the query is executed , If you don't use indexes , that MySQL It will first scan all row data in the database and filter out row records within the target range , Sort these row records and generate a temporary table , Then return the target row record of the user query through the temporary table . This process will involve the establishment of temporary tables and the sorting of row records , When there are many records in the target line , It will greatly affect the efficiency of range query .

So when you add an index , Because of the order of the index itself , When querying the range , The filtered row records have been sorted , This avoids the problems of re sorting and the need to establish temporary tables .

meanwhile , Due to the order of the underlying implementation of the index , So that when querying data , It can avoid random addressing in different sectors of the disk . After using the index, the access to data on the disk can be addressed roughly in order through disk pre reading . This is essentially based on the principle of locality .

** Locality principle ：** When a data is used , Data in the vicinity is often used immediately . The data needed during program operation is usually centralized . Due to the high efficiency of disk sequential reading ( No seek time required , Only a small amount of rotation time is required ) , Therefore, for programs with local characteristics , Disk pre reading can improve I/O efficiency .

Disk read ahead is required to read ahead every time. The length is generally an integral multiple of the page . Moreover, the database system sets the size of a node equal to one page , In this way, each node only needs one time I/O You can load it completely . The page here is realized through page memory management , The concept is briefly mentioned here .

Paging mechanism Is to divide the memory address space into several small fixed size pages , The size of each page is determined by memory . This is done to map from a virtual address to a physical address , Improve memory and disk utilization .

To sum up . The existence of index has great advantages , The main performance is as follows ：

• Indexing greatly reduces the amount of data that the server needs to scan
• Indexes can help the server avoid sorting and temporary tables
• Index can be random I/0 Into order I/0

The above three points can greatly improve the efficiency of database query , Optimize server performance . So in general , Adding efficient indexes to the database is one of the important work of database optimization .

Disadvantages of index

The existence of indexes can improve performance , Naturally, there will be additional costs in other ways .

The index itself is stored as a table , So it takes up extra storage space ; The creation and maintenance of index tables requires time and cost , This cost increases with the amount of data ; Building an index will reduce the modification of data （ Delete , add to , modify ） The efficiency of , Because you need to modify the index table as well as the data table ;

So for very small tables , The cost of using an index is greater than a direct full table scan , At this time, you don't have to use the index .

Classification of indexes

Index is a data structure used by storage engine to quickly find records , Classify according to the way of implementation , There are mainly Hash Index and B+Tree Indexes

Hash Indexes

Time complexity O(1)

principle ：

According to the specific algorithm function , Generate hash The value stores the corresponding row position , When querying, it is converted into... According to the index value hash Value hit hash Value to get the corresponding row position , Avoid full table scanning .

B+Tree Indexes

Information to be supplemented

Index type

• Single index ：

  An index contains only a single column , But you can have multiple single-column indexes in a table ,

  Single column index can be divided into ： General index , unique index , primary key

• Composite index

• Full-text index

• Spatial index

Index operation

Index view

--  View all indexes of the database 
--  grammar 
select * from mysql.`innode_index_stats` a where a.`database_name` = ' Database name ';
--  Example 1,  Inquire about mydb1 All indexes in the library 
select * from mysql.`innodb_index_stats` a where a.`database_name` = 'mydb1'; 
--  Example 2,  Inquire about mydb1 In the library student All indexes in the table 
select * from mysql.innodb_index_stats a where a.database_name = 'mydb1' and table_name = 'student';

--  View all indexes in the table  
--  grammar 
show index from table_name;
--  Example 
show index from student;

Delete index

--  The way 1
drop index  Index name  on  Table name 
--  The way 2
alter table  Table name  drop index  Index name 

Single index

Single column index can be divided into ： General index , unique index , primary key

primary key

Each table usually has its own primary key , When we create a table ,MySQL The primary key column will be automatically created An index , This is the primary key index , The primary key index is unique and cannot be NULL.

General index

MySQL Basic index type , There are no restrictions , Allows you to insert duplicate and null values in a column that defines an index , Just to query the data a little faster .

There are three ways to create a normal index ：

--  When creating a table 
create table user(
	uid int primary key,
    name varchar(50),
    index index_uid(uid) --  to uid Column to create a normal index 
);

--  The way 2- Create directly 
create index index_name on user(name); --  to user Tabular name Field creation index 

--  The way 3  Modify table structure （ Add index ）
-- alter table tableName add index index_name(cloumnname)
alter table user add index index_name(name);

unique index

The value of the index column must be unique , But you can have an empty value , If it's a composite index , The combination of column values must be unique .

--  The way 1,  When creating a table   Direct designation 
create table stu(
	sid int primary key,
	card_id varchar(20),
	name varchar(50),
	age int,
	phone varchar(11),
	unique index_card_id(card_id) --  to card_id Column creates a unique index 
);

--  The way 2, Create directly 
-- create unique index  Index name  on  Table name ( Name )
create unique index index_card_id on stu(card_id);

--  The way 3, Modify table structure 
-- alter table  Table name  add unique [ Index name ] ( Name )
alter table stu add unique index_phone (phone)

Composite index

Composite index, also known as composite index , It means when building the index Multiple fields used , Similarly, it can be established as a common index or a unique index

The leftmost principle of composite index

Format ：

--  The basic syntax for creating an index 
create index indexname on table_name(column1(length), column2(length));

--  Example ,  Create a normal index 
create index index_phone_name on stu(phone, name);

--  Example , Create unique index 
create unique index unique_index on stu(phone, sid);

What is the leftmost principle ？

First match the first index field , You must match to the first field to hit the index . If it is and Then both can hit

Example

select * from stu where name = ' full name '; --  No hit index 
select * from stu where phone = '15507551111'; --  Hit index 
select * from stu where phone = '18888888888' and name = ' full name ';  --  Hit index 
select * from stu where name = ' full name ' and phone = '1888888888';  --  Hit index 

Full-text index

The keyword of full-text index is fulltext, Not many scenes are used

• Full text index is mainly used to find keywords in text , Instead of comparing directly with the values in the index , It's more like a search engine , Query based on similarity , Not simply where The parameters of the statement match

• use like Fuzzy matching can be realized , Why full text indexing ？

  like When there is less text The index is OK , But for a large number of text data retrieval , Efficiency and performance are not very good . Full text index is in front of a large amount of text data , Effect and performance ratio like better .

summary

Full text index in mysql5.6 Previous versions , Only MyISAM Storage engine support ,5.6 in the future MyISAM and InnoDB Storage engines support full-text indexing .

Only the data of the field is char, varchar, text Its series （longtext, etc. ) Before you can build a full-text index

MySQL Full text index in , There are several key variables ,

Minimum search length and maximum search length , Less than the minimum search length and more than the maximum search length will not be indexed .

--  View the variables related to the global full-text index 
show variables like '%ft%';

Operate full text index

Create full text index

The creation method is similar to the only one above , Keywords are fulltext

Use index

Use full-text indexing and common fuzzy matching like Different , Full text index has its own syntax format , Use match and agains keyword .

Format ：

match(col1, col2, ...) against(expr [search_modifier])

Example ：

select * from article where match(content) against(' Happy New Year! '); 

Spatial index

stay MySQL5.7 Only later versions support spatial indexes .

Spatial index is to index the fields of spatial data type , Just know that you have this thing , Regular development doesn't work .